1. Technical Field
The present invention relates to a silicon nitride substrate and a method of manufacturing the same. In addition, the present invention relates to a silicon nitride circuit board and a semiconductor module using the silicon nitride substrate.
2. Description of the Related Art
Recently, in the field of an inverter for an electric vehicle or the like, power semiconductor modules (such as IGBT or a power MOSFET) capable of operating with a high voltage and a large electric current are employed. The power semiconductor module may include an insulative ceramic circuit board having a metal circuit plate on its one surface and a metal heat sink plate on the other surface. In addition, a semiconductor device is mounted on the superior surface of the metal circuit plate. The insulative ceramic substrate is bonded to the metal circuit plate and the metal heat sink plate by means of, for example, an active metal method using brazing filler metal or a direct copper bonding method in which a copper plate is directly bonded.
Since such power semiconductor module generates a large amount of heat by flowing a large amount of current, a thermal stress is generated due to a difference of thermal expansion rates between the insulative ceramic substrate and the metal circuit plate or between the insulative ceramic substrate and the metal heat sink plate. This may cause fracture that generates cracks on the insulative ceramic substrate or exfoliation of the metal circuit plate or the metal heat sink plate from the insulative ceramic substrate. The insulative ceramic substrate may be made of, for example, aluminum nitride or silicon nitride. However, since the insulative ceramic substrate made of aluminum nitride has a low mechanical strength, it may be susceptible to the cracks or exfoliation, so that may not be suitable to be used in the power semiconductor module.
In this regard, Japanese Patent Application Laid-Open (JP-A) No. 11-268958 discloses an example of a sintered silicon nitride substrate. According to the patent document, an inner layer of the substrate has a fine grain structure, and an outer layer has a combinational structure containing both coarse and fine grains, thereby improving strength and toughness. Japanese Patent Application Laid-Open (JP-A) No. 61-186257 discloses a silicon nitride ceramic structure, in which sizes of ceramic grains contained in a surface layer are larger than the sizes of the ceramic grains contained in an inner layer, thereby improving strength. In Japanese Patent Application Laid-Open (JP-A) No. 61-10069, fine powder of magnesium carbonate MgCO3 or magnesium hydroxide Mg(OH)2, which is thermally decomposed into magnesium oxide MgO, is used as a sintering additive for forming grain boundary phases, thereby obtaining a sintered body having grain boundary phases regularly diffused. Thus, strength is improved and strength difference is reduced. In Japanese Patent Application Laid-Open (JP-A) No. 2004-161605, a number of sintering additive components are previously mixed and regularly diffused, and then, silicon nitride powder as a main source material is mixed with them, so that a sintered body having high strength with suppressed agglomeration or segregation can be obtained.
However, the techniques disclosed in the aforementioned documents failed to appropriately adjust warpage and surface roughness of the silicon nitride substrate. Generally, when the warpage of the silicon nitride substrate becomes large, an adhesion property of the metal circuit plate and the metal heat sink plate is degraded, so that the metal circuit plate and the metal heat sink plate may become susceptible to exfoliation from the silicon nitride substrate due to the thermal stress generated during a cooling process from a bonding temperature (at about 800 degree C.) between the silicon nitride substrate and the metal circuit plate, and between the silicon nitride substrate and the metal heat sink plate, or a heating and cooling cycle when operating the power semiconductor module. Also, when surface roughness of the silicon nitride substrate is large, the surface adhesion property of the silicon nitride substrate with the metal circuit plate and the metal heat sink plate is degraded, so that the metal circuit plate and the metal heat sink plate may become susceptible to exfoliation from the silicon nitride substrate as described above. Accordingly, it is necessary to appropriately adjust the warpage and surface roughness, but the aforementioned documents do not disclose any technique for adjusting the warpage and the surface roughness of the silicon nitride substrate. Therefore, as described above, the warpage and the surface roughness of the silicon nitride substrate cannot be appropriately adjusted.